3 Hardware

In this chapter, you will learn about

★ primary storage/memory devices

★ secondary storage (including removable devices)

★ the benefits and drawbacks of embedded systems

★ hardware devices used as input, output and storage

★ the differences between RAM, ROM, SRAM, DRAM, PROM and EPROM

★ the use of RAM, ROM, SRAM and DRAM in a range of devices

★ monitoring and control systems

★ the use of logic gates: NOT, AND, OR, NAND, NOR and XOR

★ the construction and use of truth tables

★ the construction of logic circuits, truth tables and logic expressions

from a variety of logic information.

WHAT YOU SHOULD ALREADY KNOW

Try these five questions before you read this

chapter.

1 What is the difference between memory and

storage?

2 Why is it necessary to have both internal and

external memory/storage devices?

3 Can you recognise the memory/storage

devices on the right?

4 What is the difference between online and

offline storage?

5 What is the difference between data access

time and data transfer rate when using

memory and storage devices?

3 Hardware

Key terms

Memory cache – high speed memory external to processor

which stores data which the processor will need again.

Random access memory (RAM) – primary memory unit

that can be written to and read from.

Read-only memory (ROM) – primary memory unit that

can only be read from.

Dynamic RAM (DRAM) – type of RAM chip that needs

to be constantly refreshed.

Static RAM (SRAM) – type of RAM chip that uses

flip-flops and does not need refreshing.

Refreshed – requirement to charge a component to

retain its electronic state.

3.1 Computers and their components

▲ Figure 3.1 Memory/storage devices

457591_03_CI_AS & A_Level_CS_068-106.indd 68 26/04/19 7:27 AM

3.1 Computers and their components

3.1.1 Types of memory and storage

Computers require some form of memory and storage.

Memory is usually referred to as the internal devices which the computer can

access directly. This memory can be the user’s workspace, temporary data or

data that is key to running the computer.

Storage devices allow users to store applications, data and files. The user’s data

is stored permanently and they can change it or read it as they wish. Storage

needs to be larger than internal memory since the user may wish to store large

files (such as music files or photographic images).

Storage devices can also be removable to allow data, for example, to be

transferred between computers. Removable devices allow a user to store

important data in a different building in case of data loss.

However, all of this has become a lot less important with the advent of

technology such as ‘data drop’ (which uses Bluetooth) and cloud storage.

Internal memory includes components such as registers (which are part of the

processor). There is also memory cache (which is external to the processor);

this is used to store data which the processor will probably need to use again.

Programmable ROM (PROM) – type of ROM chip that

can be programmed once.

Erasable PROM (EPROM) – type of ROM that can be

programmed more than once using ultraviolet (UV) light.

Hard disk drive (HDD) – type of magnetic storage device

that uses spinning disks.

Latency – the lag in a system; for example, the time to

find a track on a hard disk, which depends on the time

taken for the disk to rotate around to its read-write head.

Fragmented – storage of data in non-consecutive sectors;

for example, due to editing and deletion of old data.

Removable hard disk drive – portable hard disk drive

that is external to the computer; it can be connected

via a USB part when required; often used as a device to

back up files and data.

Solid state drive (SSD) – storage media with no moving

parts that relies on movement of electrons.

Electronically erasable programmable read-only

memory (EEPROM) – read-only (ROM) chip that can

be modified by the user, which can then be erased and

written to repeatedly using pulsed voltages.

Flash memory – a type of EEPROM, particularly suited

to use in drives such as SSDs, memory cards and

memory sticks.

Optical storage – CDs, DVDs and Blu-ray

discs that

use laser light to read and write data.

Dual layering – used in DVDs; uses two recording layers.

Birefringence – a reading problem with DVDs caused

by refraction of laser light into two beams.

Binder 3D printing – 3D printing method that uses a

two-stage pass; the first stage uses dry powder and the

second stage uses a binding agent.

Direct 3D printing – 3D printing technique where print

head moves in the x, y and z directions. Layers of melted

material are built up using nozzles like an inkjet printer.

Digital to analogue converter (DAC) – needed to

convert digital data into electric currents that can drive

motors, actuators and relays, for example.

Analogue to digital converter (ADC) – needed to

convert analogue data (read from sensors, for example)

into a form understood by a computer.

Organic LED (OLED) – uses movement of electrons

between cathode and anode to produce an on-screen

image. It generates its own light so no back lighting

required.

Screen resolution – number of pixels in the horizontal

and vertical directions on a television/computer screen.

Touch screen – screen on which the touch of a finger or

stylus allows selection or manipulation of a screen image;

they usually use capacitive or resistive technology.

Capacitive – type of touch screen technology based

on glass layers forming a capacitor, where fingers

touching the screen cause a change in the electric field.

Resistive – type of touch screen technology. When

a finger touches the screen, the glass layer touches

the plastic layer, completing the circuit and causing a

current to flow at that point.

Virtual reality headset – apparatus worn on the head

that covers the eyes like a pair of goggles. It gives the

user the ‘feeling of being there’ by immersing them

totally in the virtual reality experience.

Sensor – input device that reads physical data from its

surroundings.

457591_03_CI_AS & A_Level_CS_068-106.indd 69 26/04/19 7:27 AM

3 Hardware

Figure 3.2 summarises the types of memory and storage devices covered in this

chapter.

secondary storage

hard disk drive (HDD)

solid state drive (SSD)

removable devices:

- DVD/CD/Blu-ray

- flash memory stick

- hard disk drive

primary memory

RAM

ROM

▲ Figure 3.2 Memory and storage devices

Primary memory

Primary memory is the part of computer memory which can be accessed directly

from the CPU and, as Figure 3.2 shows, contains the random access memory

(RAM) and read-only memory (ROM) memory chips. Primary memory allows

the processor to access applications and services temporarily stored in memory

locations. The structure of primary memory is shown in Figure 3.3.

Primary memory

RAM ROM

SRAM DRAM PROM EPROM EEPROM

▲ Figure 3.3 Structure of primary memory

All computer systems come with some form of RAM. These memory devices

are not really random, it refers to the fact that any memory location can be

accessed independent of which memory location was last used. Access time to

locate data is much faster in RAM than in secondary devices. RAM can also be

» written to or read from, and the data stored can be changed by the user or

by the computer

» used to store data, files, part of an application or part of the operating

system currently in use

» volatile (memory contents are lost on powering off the computer).

In general, the larger the RAM, the faster the computer will operate. In reality,

RAM never runs out of memory, it continues to operate but just becomes slower

and slower as more data is stored. As RAM becomes ‘full’, the processor has to

continually access the secondary data storage devices to overwrite old data on

RAM with new data. By increasing the RAM size, the number of times this has

to be done is considerably reduced, thus making the computer operate more

quickly.

There are currently two types of RAM technology, dynamic RAM (DRAM) and

static RAM (SRAM).

457591_03_CI_AS & A_Level_CS_068-106.indd 70 26/04/19 7:27 AM

3.1 Computers and their components

Dynamic RAM (DRAM)

Each DRAM chip consists of a number of transistors and capacitors. Each of

these parts is tiny since a single RAM chip will contain millions of capacitors

and transistors.

» Capacitors hold the bits of information (0 or 1).

» Transistors act like switches; they allow the chip control circuitry to read the

capacitor or change the capacitor’s value.

This type of RAM needs to be constantly refreshed (that is, the capacitor

needs to be re-charged every 15 microseconds otherwise it would lose its

value). If it is not refreshed, the capacitor’s charge will leak away very quickly,

leaving every capacitor with the value 0.

DRAMs have a number of advantages over SRAMs. They:

» are much less expensive to manufacture than SRAMs

» consume less power than SRAMs

» have a higher memory capacity than SRAMs.

Static RAM (SRAM)

A major difference between SRAM and DRAM is that SRAM does not need to be

constantly refreshed.

It makes use of flip flops (see Chapter 15) which hold each bit of memory.

SRAM is much faster than DRAM when it comes to data access (typically, access

time for SRAM is 25 nanoseconds and for DRAM is 60 nanoseconds).

DRAM is the most common type of RAM used in computers, but where absolute

speed is essential, for example in the processor’s memory cache, SRAM is the

preferred technology. Memory cache is a high speed portion of the memory.

It is effective because most programs access the same data or instructions

many times. By keeping as much of this information as possible in SRAM, the

computer avoids having to access the slower DRAM.

Table 3.1 summarises the differences between DRAM and SRAM.

DRAM SRAM

n consists of a number of transistors and

capacitors

n needs to be constantly refreshed

n less expensive to manufacture than SRAM

n has a higher memory capacity than SRAM

n main memory is constructed from DRAM

n consumes more power than SRAM under

reasonable levels of access, as it needs

to be constantly refreshed

n uses flip-flops to hold each bit of

memory

n does not need to be constantly

refreshed

n has a faster data access time than DRAM

n processor memory cache makes use

of SRAM

n if accessed at a high frequency, power

usage can exceed that of DRAM

▲ Table 3.1 Differences between DRAM and SRAM

Another form of primary memory is the read-only memory (ROM). This is similar

to RAM in that it shares the same random access properties, but it cannot

be written to or changed. As the name suggests, ROM is a read-only memory

device.

ROMs are

» non-volatile (the contents are not lost after powering off the computer)

» permanent memory devices (the contents cannot be changed)

▲ Figure 3.4 Two pieces

of dynamic random access

memory (DRAM)

▲ Figure 3.5 Static RAM

457591_03_CI_AS & A_Level_CS_068-106.indd 71 26/04/19 7:27 AM

3 Hardware

» often used to store data which the computer needs to access when powering

up for the first time for example, the basic input/output system (BIOS).

Table 3.2 summarises the main differences between RAM and ROM.

RAM ROM

n temporary memory device

n volatile memory

n can be written to and read from

n used to store data, files, programs, part

of OS currently in use

n can be increased in size to improve

operational speed of a computer

n permanent memory device

n non-volatile memory device

n data stored cannot be altered

n sometimes used to store BIOS and other

data needed at start up

▲ Table 3.2 Differences between RAM and ROM

PROM and EPROM

A programmable read-only memory (PROM) is a type of ROM chip that

can be altered once. A PROM is made up of a matrix of fuses. Programming

a PROM requires the use of a PROM writer which uses an electric current to

alter specific cells by ‘burning’ fuses in the matrix. Due to the method of

programming (writing), a PROM can only be written to once. They are often

used in mobile phones and in RFID tags.

An erasable programmable read-only memory (EPROM) is different to a PROM

because they use floating gate transistors and capacitors rather than fuses.

Ultra violet (UV) light is used to program an EPROM through a quartz window.

They are used in applications which are under development, such as the

programming of new games consoles.

Embedded systems

Embedded systems involve installing microprocessors into devices to enable

operations to be controlled in a more efficient way. Devices such as cookers,

refrigerators and central heating systems can now all be activated by a

web-enabled device (such as a mobile phone or tablet). The time a central

heating system switches on or off and the temperature can all be set from an

app on a mobile phone from anywhere in the world.

There are pros and cons of devices being controlled in this manner, as shown in

Table 3.3.

Pros of embedded systems Cons of embedded systems

n small in size and therefore easy to

fit into devices

n relatively low cost to make

n usually dedicated to one task,

making for simple interfaces

and often no requirement of an

operating system

n consume very little power

n very fast reaction to changing input

(operate in real time)

n with mass production comes

reliability

n difficult to upgrade devices to take

advantage of new technology

n troubleshooting faults in the device

becomes a specialist task

n although the interface can appear to be

simple, in reality it can be more confusing

(changing the time on a cooker clock can

require several steps, for example)

n any device that can be accessed over the

internet is also open to hackers, viruses,

and so on

n due to the difficulty in upgrading and fault

finding, devices are often just thrown away

rather than being repaired (wasteful)

▲ Table 3.3 Pros and cons of controlling devices with embedded systems

457591_03_CI_AS & A_Level_CS_068-106.indd 72 26/04/19 7:27 AM

3.1 Computers and their components

Secondary storage devices

Secondary storage includes storage devices that are not directly accessible by

the CPU. They are non-volatile devices which allow data to be stored as long as

required by the user. This type of storage is much larger than primary memory,

but data access time is considerably slower than RAM and ROM. All applications,

the operating system, device drivers and general files (for example, documents,

photos and music) are stored on secondary storage. The following section

discusses the various types of secondary storage that can be found on the

majority of computers. Secondary storage devices fall into three categories:

magnetic, solid state and optical.

Hard disk drives (HDD)

Hard disk drives (HDD) are still one of the most common methods used to

store data on a computer.

Data is stored in a digital format on the magnetic surfaces of the disks

(or platters, as they are frequently called). The hard disk drive will have a

number of platters which can spin at about 7000 times a second. A number

of read-write heads can access all of the surfaces in the disk drive. Normally

each platter will have two surfaces which can be used to store the data.

These read-write heads can move very quickly – typically they can move from

the centre of the disk to the edge of the disk (and back again) 50 times a

second.

Data is stored on the surface in sectors and tracks.

A sector on a given track will contain a fixed number of bytes.

Unfortunately, hard disk drives have very slow data access when compared

to, for example, RAM. Many applications require the read-write heads to

constantly seek for the correct blocks of data; this means a large number of

head movements. The effects of latency then become very significant. Latency

is defined as the time it takes for a specific block of data on a data track to

rotate around to the read-write head.

Users will sometimes notice the effect of latency when they see messages such

as, ‘Please wait’ or, at its worst, ‘not responding’.

When a file or data is stored on an HDD, the required number of sectors needed

to store the data will be allocated. However, the sectors allocated may not be

adjacent to each other. Through time, the HDD will undergo numerous deletions

and editing, which leads to sectors becoming increasingly fragmented,

resulting in a gradual deterioration of the HDD performance (in other words, it

takes longer and longer to access data). Defragmentation software can improve

on this situation by ‘tidying up’ the disk sectors.

An HDD is a direct access device; however, data in a given sector will be read

sequentially.

EXTENSION ACTIVITY 3A

Describe how ROM and RAM chips could be used in:

a) a microwave oven

b) a refrigerator

c) a remote-controlled model aeroplane (the movement of the aeroplane is

controlled by a hand-held device).

track

sector

▲ Figure 3.6 Tracks and

sectors on a hard disk drive

457591_03_CI_AS & A_Level_CS_068-106.indd 73 26/04/19 7:27 AM

3 Hardware

Removable hard disk drives are essentially HDDs that are external to the

computer and can be connected to the computer using one of the USB ports. In

this way, they can be used as back-up devices or as another way of transferring

files between computers.

Solid state drives (SSD)

Latency is an issue in HDDs, as discussed earlier. Solid state drives (SSD)

reduce this issue considerably. They have no moving parts and all data is

retrieved at the same rate. They do not rely on magnetic properties. The most

common type of solid state storage devices store data by controlling the

movement of electrons within NAND chips. The data is stored as 0s and 1s in

millions of tiny transistors (at each junction one transistor is called a floating

gate and the other is called a control gate) within the chip. This effectively

produces a non-volatile rewritable memory.

However, a number of solid state storage devices sometimes use electronically

erasable PROM (EEPROM) technology. The main difference is the use of NOR chips

rather than NAND. This makes them faster in operation but devices using EEPROM

are considerably more expensive than those that use NAND technology. EEPROM also

allows data to be read or erased in single bytes at a time. Use of NAND only allows

blocks of data to be read or erased. This makes EEPROM technology more useful in

certain applications where data needs to be accessed or erased in byte-size chunks.

Because of the cost implications, the majority of solid state storage devices

use NAND technology. The two are usually distinguished by the terms flash

memory (use NAND) and EEPROM (use NOR).

So, what are the main benefits of using an SSD rather than an HDD?

Solid state drives

» are more reliable (no moving parts to go wrong)

» are considerably lighter (which makes them suitable for laptops)

» do not have to ‘get up to speed’ before they work properly

» have a lower power consumption

» run much cooler than HDDs (both these points again make them very

suitable for laptop computers)

» are very thin (because they have no moving parts)

» access data considerably faster.

The main drawback of SSD is the still unknown longevity of the technology.

Most solid state storage devices are conservatively rated at only 20 GB write

operations per day over a three year period – this is known as SSD endurance.

For this reason, SSD technology is not commonly used in servers, for example,

where a huge number of write operations take place every day. However, this

issue is being addressed by a number of manufacturers to improve the durability

of these solid state systems and they are rapidly becoming more common in

applications such as servers and cloud storage devices.

Note that it is also not possible to over-write existing data on a flash memory

device; it is necessary to first erase the old data and then write the new data

at the same location.

EXTENSION ACTIVITY 3B

The length of a track on each disk in an HDD disk pack becomes much

shorter towards the centre of the disk. Find out how manufacturers have

overcome this issue with regards to disk data capacity and data access time.

457591_03_CI_AS & A_Level_CS_068-106.indd 74 26/04/19 7:27 AM

3.1 Computers and their components

Memory sticks/flash memories (also known as pen drives) use solid state

technology. They usually connect to the computer through the USB port. Their

main advantage is that they are very small, lightweight devices which make

them suitable for transferring files between computers. They can also be used

as small back-up devices for music or photo files, for example.

Complex or expensive software, such as an expert system, will often use a

memory stick as a dongle. The dongle contains additional files which are

needed to run the software. Without this dongle, the software will not work

properly. It therefore prevents illegal or unauthorised use of the software, and

also prevents copying of the software since, without the dongle, it is useless.

Optical media: CDs, DVDs and Blu-ray discs

CDs and DVDS are described as optical storage devices. Laser light is used to

read data from, and write data onto, the surface of a disk.

▲ Figure 3.7 CDs and DVDs use a single, spiral track

Both CDs and DVDs use a thin layer of metal alloy or light-sensitive organic

dye to store the data. As shown in Figure 3.7, both systems use a single, spiral

track which runs from the centre of the disk to the edge. When a disk spins, the

optical head moves to the point where the laser beam ‘contacts’ the disk surface

and follows the spiral track from the centre outwards. As with an HDD, a CD/DVD

is divided into sectors allowing direct access of data. Also, as in the case of an

HDD, the outer part of the disk runs faster than the inner part of the disk.

The data is stored in ‘pits’ and ‘bumps’ on the spiral track. A red laser is used to

read and write the data. CDs and DVDs can be designated R (write once only) or

RW (can be written to or read from many times).

DVD technology is slightly different to that used in CDs. One of the main

differences is the use of dual layering which considerably increases the

storage capacity. This means that there are two individual recording

layers. Two layers of a standard DVD are joined together with a transparent

(polycarbonate) spacer, and a very thin reflector is sandwiched between the

two layers. Reading and writing of the second layer is done by a red laser

focusing at a fraction of a millimetre difference compared to the first layer.

single spiral track runs

from the centre to outer

part of disk

pits or bumps

EXTENSION ACTIVITY 3C

The outer part of an optical disk runs faster than the inner part of the disk.

Find out how manufacturers have overcome this issue with regards to disk

data capacity and data access time.

457591_03_CI_AS & A_Level_CS_068-106.indd 75 26/04/19 7:27 AM

3 Hardware

polycarbonate layer first layer

second layer

laser reads

layer 2

laser reads

layer 1

polycarbonate layer

▲ Figure 3.8 Dual layering in a DVD

Standard, single layer DVDs still have a larger storage capacity than CDs because

the ‘pit’ size and track width are both smaller. This means that more data can be

stored on the DVD surface. DVDs use lasers with a wavelength of 650 nanometres;

CDs use lasers with a wavelength of 780 nanometres. The shorter the wavelength

of the laser light, the greater the storage capacity of the medium.

» Blu-ray discs are another example of optical storage media. However, they

are fundamentally different to DVDs in their construction and in the way

they carry out read-write operations.

» Blu-ray uses a blue laser, rather than a red laser, to carry out read and write

operations; the wavelength of blue light is only 405 nanometres (compared

to 650 nm for red light).

» Using blue laser light means that the ‘pits’ and ‘bumps’ can be much smaller;

consequently, a Blu-ray can store up to five times more data than a DVD.

» Blu-ray uses a single 1.1 mm thick polycarbonate disk; DVDs use a sandwich

of two 0.6 mm thick disks.

» Using two sandwiched layers can cause birefringence (light is refracted into

two separate beams causing reading errors); because Blu-ray uses only one

layer, the discs do not suffer from birefringence.

» Blu-ray discs automatically come with a secure encryption system which

helps to prevent piracy and copyright infringement.

Table 3.4 summarises the main differences between CDs, DVDs and Blu-ray.

disk type

laser

colour

wavelength

of laser light disk construction

track pitch

(distance

between tracks)

CD red 780 nm single 1.2 mm

polycarbonate layer

1.60 µm

DVD red 650 nm two 0.6 mm

polycarbonate layers

0.74 µm

Blu-ray blue 405 nm single 1.1 mm

polycarbonate layer

0.30 µm

nm = 10

−9

metres

µm = 10

−6

metres

▲ Table 3.4 Main differences between CDs, DVDs and Blu-ray

All these optical storage media are used as back-up systems (for photos,

music and multimedia files). This also means that CDs and DVDs can be used

to transfer files between computers. Manufacturers sometimes supply their

software (such as printer drivers) on CDs and DVDs. When the software is

supplied in this way, the disk is usually in a read-only format.

The most common use of DVD and Blu-ray is the supply of movies or games. The

memory capacity of CDs is not big enough to store most movies.

457591_03_CI_AS & A_Level_CS_068-106.indd 76 26/04/19 7:27 AM

3.1 Computers and their components

3.1.2 Input and output devices

This section will consider laser printers, inkjet printers, 3D printers, speakers,

microphones, screens and sensors.

Laser printers

Laser printers use dry powder ink rather than liquid ink and make use of the

properties of static electricity to produce the text and images. Unlike inkjet

printers, for example, laser printers print the whole page in one go. Colour laser

printers use four toner cartridges – blue, cyan, magenta and black. Although

the actual technology is different to monochrome printers, the printing method

is similar, but colour dots are used to build up the text and images.

When a user wishes to print a document using a laser printer, the following

sequence of events takes place.

Stage Description of what happens

1 data from the document is sent to a printer driver

2 printer driver ensures that the data is in a format that the chosen printer

can understand

3 check is made by the printer driver to ensure that the chosen printer is

available to print (is it busy? is it off-line? is it out of ink? and so on)

4 data is sent to the printer and stored in a temporary memory known as a

printer buffer

5 printing drum given a positive charge. As this drum rotates, a laser beam

scans across it removing the positive charge in certain areas, leaving

negatively charged areas which exactly match the text/images of the page

to be printed

6 drum is coated with positively charged toner (powdered ink). Since the toner is

positively charged, it only sticks to the negatively charged parts of the drum

7 negatively charged sheet of paper is rolled over the drum

8 toner on the drum sticks to the paper to produce an exact copy of the page

sent to the printer

9 to prevent the paper sticking to the drum, the electric charge on the paper

is removed after one rotation of the drum

10 the paper goes through a fuser (a set of heated rollers), where the heat

melts the ink so that it fixes permanently to the paper

11 a discharge lamp removes all the electric charge from the drum so it is

ready to print the next page

▲ Table 3.5 Sequence to print using a laser printer

EXTENSION ACTIVITY 3D

A recent development is PRAM (parameter RAM) or PCRAM (phase-change

RAM) which utilises chalogenide glass. This is glass containing elements

such as sulphur, antimony, selenium, germanium or tellurium. Chalogenide

compounds used in PRAMs/PCRAMs can be changed between the

amorphous (glass-like) state and crystalline state, which changes the optical

and electrical properties allowing the storage of data when used as a film on

the surface of optical media.

Find out more about this technology and determine whether this could result

in the demise of the current solid state removable devices.

▲ Figure 3.9 A laser printer

457591_03_CI_AS & A_Level_CS_068-106.indd 77 26/04/19 7:27 AM

3 Hardware

Inkjet printers

Inkjet printers are made up of

» a print head consisting of nozzles that spray droplets of ink onto the paper

to form characters

» an ink cartridge or cartridges; either one cartridge for each colour (blue,

yellow and magenta) and a black cartridge, or one single cartridge

containing all three colours and black (note: some systems use six colours)

» a stepper motor and belt which moves the print head assembly across the

page from side to side

» a paper feed which automatically feeds the printer with pages as they are

required.

The ink droplets are currently produced using one of two technologies: thermal

bubble or piezoelectric.

Thermal bubble – tiny resistors create localised heat which makes the ink

vaporise. This causes the ink to form a tiny bubble, as the bubble expands

some of the ink is ejected from the print head onto the paper. When the

bubble collapses, a small vacuum is created which allows fresh ink to

be drawn into the print head. This continues until the printing cycle is

completed.

Piezoelectric – a crystal is located at the back of the ink reservoir for each

nozzle. The crystal is given a tiny electric charge which makes it vibrate. This

vibration forces ink to be ejected onto the paper and at the same time more ink

is drawn in for further printing.

When a user wishes to print a document using an inkjet printer, the following

sequence of events takes place. Whatever technology is used, the basic steps in

the printing process are the same.

Stage Description of what happens

1 data from the document is sent to a printer driver

2 printer driver ensures that the data is in a format that the chosen printer

can understand

3 check is made by the printer driver to ensure that the chosen printer is

available to print (is it busy? is it off-line? is it out of ink? and so on)

4 data is sent to the printer and stored in a temporary memory known as a

printer buffer

5 a sheet of paper is fed into the main body of the printer. A sensor detects

whether paper is available in the paper feed tray – if it is out of paper (or

the paper is jammed), an error message is sent back to the computer

6 as the sheet of paper is fed through the printer, the print head moves from

side to side across the paper printing the text or image. The four ink colours

are sprayed in their exact amounts to produce the desired final colour

7 at the end of each full pass of the print head, the paper is advanced very

slightly to allow the next line to be printed. This continues until the whole

page has been printed

8 if there is more data in the printer buffer, then the whole process from stage

5 is repeated until the buffer is empty

9 once the printer buffer is empty, the printer sends an interrupt to the processor

in the computer, which is a request for more data to be sent to the printer. The

process continues until the whole of the document has been printed

▲ Table 3.6 Sequence to print using a laser printer

▲ Figure 3.10 An inkjet

printer

457591_03_CI_AS & A_Level_CS_068-106.indd 78 26/04/19 7:27 AM

3.1 Computers and their components

3D printers

▲ Figure 3.11 A 3D printer

3D printers are used to produce working, solid objects. They are primarily based

on inkjet and laser printer technology. The solid object is built up layer by layer

using materials such as powdered resin, powdered metal, paper or ceramic.

The artificial bone framework in Figure 3.12 was made from many layers (100 µm

thick) of powered metal using a technology known as binder 3D printing.

Various types of 3D printers exist; they range from the size of a microwave

oven up to the size of a small car.

3D printers use additive manufacturing (the object is built up layer by layer);

this is in contrast to the more traditional method of subtractive manufacturing

(removal of material to make the object). For example, making a statue using

a 3D printer would involve building it up layer by layer using powdered stone

until the final object was formed. The subtractive method would involve

carving the statue out of solid stone (removing the stone not required) until

the final item was produced. Similarly, CNC machining removes metal to form

an object; 3D printing would produce the same item by building up the object

from layers of powdered metal.

Direct 3D printing uses inkjet technology; a print head can move left to right

as in a normal printer. However, the print head can also move up and down to

build up the layers of an object.

Binder 3D printing is similar to direct 3D printing. However, this method uses

two passes for each of the layers; the first pass sprays dry powder and then on

the second pass a binder (a type of glue) is sprayed to form a solid layer.

Newer technologies use lasers and UV light to harden liquid polymers; this

further increases the diversity of products which can be made.

▲ Figure 3.12 Artificial

bone framework made

using an industrial

3D printer

457591_03_CI_AS & A_Level_CS_068-106.indd 79 26/04/19 7:27 AM

3 Hardware

Speakers and microphones

Speakers

Digitised sound stored in a file on a computer can be converted into sound as

follows:

» The digital data is first passed through a digital to analogue converter (DAC)

where it is converted into an electric current.

» This is then passed through an amplifier (since the current generated

by the DAC will be small) to create a current large enough to drive a

loudspeaker.

» This electric current is then fed to a loudspeaker where it is converted into

sound.

The following schematic shows how this is done.

▲ Figure 3.13 Digital to analogue conversion

As Figure 3.13 shows, if the sound is stored in a computer file, it must first

pass through a digital to analogue converter (DAC) to convert the digital

data into an electric current which can be used to drive the loudspeaker.

Figure 3.14 shows how a loudspeaker can convert electric signals into sound

waves.

sound waves

plastic or

paper cone

sound waves

produced

coil of wire

wrapped

around an

iron core

electric current fed to wire

permanent

magnet

▲ Figure 3.14 Diagram showing how a loudspeaker works

» When an electric current flows through a coil of wire that is wrapped around

an iron core, the core becomes a temporary electromagnet; a permanent

magnet is also positioned very close to this electromagnet.

» As the electric current through the coil of wire varies, the induced magnetic

field in the iron core also varies. This causes the iron core to be attracted

towards the permanent magnet and as the current varies this will cause the

iron core to vibrate.

» Since the iron core is attached to a cone (made from paper or thin synthetic

material), this causes the cone to vibrate, producing sound.

The rate at which the DAC can translate the digital output into analogue

voltages is known as the sampling rate. If the DAC is a 16-bit device, then it

speaker

amplifer

DAC

1 0 0 1 0 1 0 1 0 1 1

457591_03_CI_AS & A_Level_CS_068-106.indd 80 26/04/19 7:27 AM

3.1 Computers and their components

can accept numbers between +32 767 (2

– 1) and –32 768 (2

); the digital

value containing all zeros is ignored.

Microphones

Microphones are either built into the computer or are external devices

connected through the USB port or through wireless connectivity.

Figure 3.15 shows how a microphone can convert sound waves into an electric

current. The current produced can either be stored as sound (on, for example, a

CD), amplified and sent to a loudspeaker, or sent to a computer for storage.

sound waves

cone

diaphragm

coil wrapped around

a permanent magnet

output from

the microphone

▲ Figure 3.15 Diagram of how a microphone works

» When sound is created, it causes the air to vibrate.

» When a diaphragm in the microphone picks up the air vibrations, the

diaphragm also begins to vibrate.

» A copper coil is wrapped around a permanent magnet and the coil is

connected to the diaphragm using a cone. As the diaphragm vibrates, the

cone moves in and out causing the copper coil to move backwards and

forwards.

» This forwards and backwards motion causes the magnetic field around the

permanent magnet to be disturbed, inducing an electric current.

» The electric current is then either amplified or sent to a recording device.

The electric current is analogue in nature.

The electric current output from the microphone can also be sent to a computer

where a sound card converts the current into a digital signal which can then be

stored in the computer. The following diagram shows what happens when the

word ‘hut’ is picked up by a microphone and is converted into digital values:

▲ Figure 3.16 Analogue to digital conversion

Look at Figure 3.16. The word ‘hut’ (in the form of a sound wave) has been

picked up by a microphone; this is then converted using an analogue to digital

converter (ADC) into digital values which can then be stored in a computer or

manipulated as required using appropriate software.

Screens

Screens are used to show the output from a computer. Modern screens use an LCD,

backlit with LEDs or the newer organic light emitting diode (OLED) technology.

1000 0001

0001 1110

1000 1110

0001 1100

1100 1100

1101 1110

sound wave for ‘HUT’ digital value after conversion

457591_03_CI_AS & A_Level_CS_068-106.indd 81 26/04/19 7:27 AM

3 Hardware

Figure 3.17 shows a simplified form of how OLED technology works.

▲ Figure 3.17 Simplified form of how OLED technology works

OLEDs use organic materials (made up of carbon compounds) to create flexible

semiconductors. Organic films are sandwiched between two charged electrodes

(one is a metallic cathode and the other a glass anode). When an electric field

is applied to the electrodes, they give off light. This means that no form of

back lighting is required. This allows for very thin screens. It also means that

there is no longer a need to use LCD technology, since OLED is a self-contained

system.

Screen displays are based on the pixel (the smallest picture element) concept

where each screen pixel is made up of three sub-pixels, which are red, green

and blue. By varying the intensity of the three sub-pixels, it is possible to

generate millions of colours. The greater the number of pixels on a screen,

the greater is the screen resolution (the number of pixels which can be

viewed horizontally and vertically on screen; for example, 1680 × 1080

pixels). LCD and OLED screens use this type of pixel matrix to make up the

picture.

The ‘purple’ pixel is made up of a combination of

three sub-pixels, which are red, green and blue, in the

required intensity, to ‘fool’ the eye into seeing a

purple dot on the screen. The whole screen is filled

with thousands of these tiny pixels.

▲ Figure 3.18 The pixel matrix

Touch screens (which act as both input and output devices) also make use

of LCD and OLED technology. They are particularly used in mobile phones and

tablets.

We shall now consider LCD capacitive and resistive touch screen technologies.

Capacitive

» Made up of many layers of glass that act like a capacitor creating electric

fields between the glass plates in layers.

» When the top glass layer is touched, the electric current changes and the

coordinates where the screen was touched are determined by an on board

microprocessor.

Benefits

Medium cost technology.

» Screen visibility is good even in strong sunlight.

» Permits multi-touch capability.

» Screen is very durable; it takes a major impact to break the glass.

negative charges

positive charges

glass or plastic top layer

metallic cathode (negative charge)

emissive layer

conductive layer

glass anode (positive charge)

glass or plastic bottom layer

457591_03_CI_AS & A_Level_CS_068-106.indd 82 26/04/19 7:27 AM

3.1 Computers and their components

Drawbacks

Only allows use of bare fingers as the form of input; although the latest

screens permit the use of a special stylus to be used.

Resistive

» Makes use of an upper layer of polyester (a form of plastic) and a bottom

layer of glass.

» When the top polyester layer is touched, the top layer and bottom layer

complete a circuit.

» Signals are then sent out, which are interpreted by a microprocessor

and the calculations determine the coordinates of where the screen was

touched.

Benefits

Relatively inexpensive technology.

» Possible to use bare fingers, gloved fingers or stylus to carry out an input

operation.

Drawbacks

Screen visibility is poor in strong sunlight.

» Does not permit multi-touch capability.

» Screen durability is only fair; it is vulnerable to scratches and the screen

wears out through time.

Virtual headsets

Virtual reality has now been around for many years and has many applications.

For example, it is possible to ‘walk around’ inside dangerous areas – such as a

nuclear power plant – without actually being there.

It allows engineers to plan modifications or repairs to a plant in complete

safety and to try out different scenarios first before implementing them. One

of the devices used is a virtual reality headset which gives the engineer the

feeling of being there. We will now describe how these devices work.

» Video is sent from a computer to the headset (either using an HDMI cable or

a smartphone fitted into the headset).

» Two feeds are sent to an LCD/OLED display (sometimes two screens are

used, one for the left side of the image and one for the right side of the

image); lenses placed between the eyes and the screen allow for focusing

and reshaping of the image/video for each eye, thus giving a 3D effect and

adding to the realism.

» Most headsets use 110° field of view which is enough to give a pseudo 360°

surround image/video.

» A frame rate of 60 to 120 images per second is used to give a true/realistic

image.

» As the user moves their head (up and down or left to right), a series of

sensors and/or LEDs measure this movement, which allows the image/video

on the screen to react to the user’s head movements (sensors are usually

gyroscopic or accelerometers; LEDs are used in conjunction with mini

cameras to further monitor head movements).

» Headsets also use binaural sound (surround sound) so that the speaker

output appears to come from behind, from the side or from a distance,

giving very realistic 3D sound.

457591_03_CI_AS & A_Level_CS_068-106.indd 83 26/04/19 7:27 AM

3 Hardware

» Some headsets also use infrared sensors to monitor eye movement (in

addition to head movement), which allows the depth of field on the screen

to be more realistic; an example of this is to make objects in the foreground

appear fuzzy when the user’s eyes indicate they are looking into the

distance (and vice versa).

Sensors

Sensors are input devices which read or measure physical properties, such as

temperature, pressure, acidity, and so on.

Real data is analogue in nature – this means it is constantly changing and

does not have a discrete value. Analogue data usually requires some form of

interpretation, for example, the temperature shown on a mercury thermometer

requires the user to look at the height of the mercury to work out the

temperature. The temperature, therefore, can have an infinite number of values

depending on the precision of how the height of the mercury is measured.

Equally, an analogue clock requires the user to look at the hands on the clock

face. The area swept out by the hands allows the number of hours and minutes

to be interpreted. There are many other examples.

Computers cannot make any sense of these physical quantities and the data

needs to be converted into a digital format. This is usually achieved by an

analogue to digital converter (ADC). This device converts physical values into

discrete digital values.

ADC

digital dataanalogue data

1 0 0 1 1 1 0 0 ...

▲ Figure 3.19 Converting analogue data into digital data

When a computer is used to control devices, such as a motor or a valve,

it is often necessary to use a digital to analogue converter (DAC), since

these devices need analogue data to operate in many cases. Frequently,

an actuator is used in these control applications. Although these are

technically output devices, they are mentioned here since they are an

integral part of the control system. An actuator is an electromechanical

device such as a relay, solenoid or motor. Note that a solenoid is an example

of a digital actuator as part of the device is connected to a computer which

opens and closes a circuit as required. When energized, the solenoid may

operate a plunger or armature to control, for example, a fuel injection

system. Other actuators, such as motors and valves, may require a DAC so

that they receive an electric current rather than a simple digital signal

direct from the computer.

Notice the importance of (positive) feedback, which is where the output

from the system can affect the next input. This is due to the fact that sensor

readings may cause the microprocessor to alter a valve or a motor, for example,

which will then change the next reading taken by the sensor. So the output

from the microprocessor will impact on the next input received as it attempts

to bring the system within the desired parameters.

457591_03_CI_AS & A_Level_CS_068-106.indd 84 26/04/19 7:27 AM

3.1 Computers and their components

Table 3.7 shows a number of common sensors and examples of their

applications.

Sensor Example applications

temperature

n control a central heating system

n control/monitor a chemical process

n control/monitor temperature in a greenhouse

moisture/humidity

n control/monitor moisture/humidity levels in soil/air in a

greenhouse

n monitor dampness levels in an industrial application (for

example, monitor moisture in a paint spray booth in a car

factory)

light

n switch street lighting on at night and off during the day

n monitor/control light levels in a greenhouse

n switch on car headlights when it gets dark

infrared/motion

n turn on windscreen wipers on a car when it rains

n detect an intruder in a burglar alarm system

n count people entering or leaving a building

pressure

n detect intruders in a burglar alarm system

n check weight (such as the weight of a vehicle)

n monitor/control a process where gas pressure is important

acoustic/sound

n pick up noise levels (such as footsteps or breaking glass) in a

burglar alarm system

n detect noise of liquids dripping from a pipe

gas (such as O

or CO

)

n monitor pollution levels in a river or air

n measure O

and CO

levels in a greenhouse

n check for CO

or NO

leaks in a power station

n monitor/control acidity/alkalinity levels in soil

n monitor pollution in rivers

magnetic field

n detect changes in in cell phones, CD players, and so on

n used in anti-lock braking systems in motor vehicles

▲ Table 3.7 Common sensors and examples of applications

Sensors are used in both monitoring and control applications. There is a subtle

difference between how these two methods work. The flowchart (Figure 3.21

overleaf) shows a simplification of the process.

457591_03_CI_AS & A_Level_CS_068-106.indd 85 26/04/19 7:27 AM

3 Hardware

sensors send signals to the

microprocessor or computer

the signals are converted to

digital (if necessary) using an

analogue to digital converter

(ADC)

the microprocessor or computer

analyses the data received

by checking it against stored

values

if new data is outside the

acceptable range, a warning

message is sent to a screen

or an alarm is activated

if the new data is outside the

acceptable range, the

microprocessor or computer

sends signals to control valves,

motors, and so on

the microprocessor or computer

has no effect on what is

being monitored – it is simply

‘watching’ the process

the output from the system

affects the next set of inputs

from the sensors

feedback loop

control systemmonitoring system

▲ Figure 3.20 Sensors for monitoring and controlling systems

Table 3.8 shows some examples of monitoring and control applications of

sensors.

Examples of monitoring Examples of control

n monitoring a patient in a hospital

for vital signs such as heart rate,

temperature, and so on

n checking for intruders in a burglar alarm

system

n checking the temperature levels in a car

engine

n monitoring pollution levels in a river

n turning street lights on at night and

turning them off again during daylight

n controlling the temperature in a central

heating/air conditioning system

n controlling the traffic lights at a road

junction

n operating anti-lock brakes on a car

when necessary

n controlling the environment in a

greenhouse

▲ Table 3.8 Examples of monitoring and control applications of sensors

One of the most common uses of sensors in modern times is in the monitoring

and control of a number of functions in motor vehicles and aeroplanes. Look at

Figure 3.21 showing a typical modern car and its many sensors used to control

or monitor several functions.

457591_03_CI_AS & A_Level_CS_068-106.indd 86 26/04/19 7:27 AM

3.1 Computers and their components

ABS

engine

management

front airbag sensors

collision

avoidance

system

rear lighting control

front lighting

control

▲ Figure 3.21 Sensors on a typical modern car

ACTIVITY 3A

1 a) i) Describe three differences between RAM and ROM.

ii) Compare the relative advantages and disadvantages of SRAM and

DRAM.

Include examples of where each type of memory would be used in

a computer.

Below is an in-depth look at just one of the sensor systems labelled on

Figure 3.21.

Anti-lock braking systems (on cars)

Anti-lock braking systems (ABS) on cars use magnetic field sensors to stop the

wheels locking up on the car if the brakes have been applied too sharply.

» When one of the car wheels rotates too slowly (it is locking up), a magnetic

field sensor sends data to a microprocessor.

» The microprocessor checks the rotation speed of the other three wheels.

» If they are different (rotating faster), the microprocessor sends a signal to

the braking system and the braking pressure to the affected wheel is reduced.

» The wheel’s rotational speed is then increased to match the other wheels.

» The checking of the rotational speed using these magnetic field sensors is

done several times a second and the braking pressure to all the wheels can

be constantly changing to prevent any of the wheels locking up under heavy

braking.

» This is felt as a ‘judder’ on the brake pedal as the braking system is constantly

switched off and on to equalise the rotational speed of all four wheels.

» If one of the wheels is rotating too quickly, braking pressure is increased to

that wheel until it matches the other three.

457591_03_CI_AS & A_Level_CS_068-106.indd 87 26/04/19 7:27 AM

3 Hardware

b) Secondary storage can be magnetic, optical or solid state.

Describe two features of each type of storage which differentiates it

from the other two types.

2 a) Explain the main differences in operation of a laser printer compared

with an inkjet printer.

b) i) Name one application of a laser printer and one application of an

inkjet printer.

ii) For each of your named applications in part b) i), give a reason why

the chosen printer is the most suitable.

3 An art gallery took several photographs of a valuable, fragile painting.

The images were sent to a computer where they were processed by a 3D

printing application. A 3D printout of the painting was produced showing

the texture of the oil paint, canvas and any flaws in the painting.

Give reasons why the art gallery would wish to make this 3D replica.

4 The following diagram shows a schematic of a microprocessor-controlled

street lighting system.

sensor

ADC

DAC

street

light

microprocessor

The microprocessor is used to control the operation of the street lamp.

The lamp is fitted with a light sensor which constantly sends data to the

microprocessor. The data value from the sensor changes according to

whether it is sunny, cloudy, raining, night time, and so on.

Describe how the microprocessor would be used to automatically switch

on the light at night and switch it off again when it becomes light. Include

a feature to stop the light constantly flickering on and off when it becomes

overcast or cars go past with full headlights at night.

EXTENSION ACTIVITY 3E

1 Look at this simplified diagram of a keyboard;

the letter H has been pressed. Explain:

a) how pressing the letter H has been

recognised by the computer

b) how the computer manages the very slow

process of inputting data from a keyboard.

2 a) Describe how these types of pointing devices

work.

i) Mechanical mouse

ii) Optical mouse

b) Connectivity between mouse and computer

can be through USB cable or wireless.

Explain these two types of connectivity.

conductive layers

letter H has been pressed and

now makes contact with bottom

conductive layer

letter H

interpreted

by computer

insulating layer

457591_03_CI_AS & A_Level_CS_068-106.indd 88 26/04/19 7:27 AM

3.2 Logic gates and logic circuits

3.2.1 Logic gates

Electronic circuits in computers, many memories and controlling devices are

made up of thousands of logic gates. Logic gates take binary inputs and

produce a binary output. Several logic gates combined together form a logic

circuit and these circuits are designed to carry out a specific function. The

checking of the output from a logic gate or logic circuit can be done using a

truth table.

This section will consider the function and role of logic gates, logic circuits

and truth tables. A number of possible applications of logic circuits will also

be considered. A reference to Boolean algebra will be made throughout this

section, although this is covered in more depth in Chapter 15.

Six different logic gates will be considered in this section.

NOT gate AND gate OR gate

XOR gateNOR gateNAND gate

▲ Figure 3.22 Six types of logic gate

EXTENSION ACTIVITY 3F

Another new screen technology is known as quantum LED (QLED), which is

in direct competition with organic (LED). Look at this statement:

‘QLED televisions are simply LED televisions that use quantum dots to

enhance their overall performance in key picture quality areas.’

Find out the main differences between QLED and OLED technologies.

3.2 Logic gates and logic circuits

Key terms

Logic gates – electronic circuits which rely on

‘on/off’ logic. The most common ones are NOT, AND,

OR, NAND, NOR and XOR.

Logic circuit – formed from a combination of logic gates

and designed to carry out a particular task. The output

from a logic circuit will be 0 or 1.

Truth table – a method of checking the output from

a logic circuit. They use all the possible binary input

combinations depending on the number of inputs;

for example, two inputs have 2

(4) possible binary

combinations, three inputs will have 2

(8) possible

binary combinations, and so on.

Boolean algebra – a form of algebra linked to logic

circuits and based on TRUE and FALSE.

457591_03_CI_AS & A_Level_CS_068-106.indd 89 02/05/19 12:35 PM

3 Hardware

3.2.2 Truth tables

Truth tables are used to trace the output from a logic gate or logic circuit. The

NOT gate is the only logic gate with one input; the other five gates have two

inputs. When constructing truth tables, all possible combinations of 1s and 0s

which can be input are considered. For the NOT gate (one input) there are only

(2) possible binary combinations. For all other gates (two inputs), there are

(4) possible binary combinations.

For logic circuits, the number of inputs can be more than 2; for example, three

inputs give a possible 2

(8) binary combinations. And for four inputs, the

number of possible binary combinations is 2

(16). It is clear that the number

of possible binary combinations is a multiple of the number 2 in every case.

Table 3.9 summarises this.

Inputs Inputs Inputs

A B A B C A B C D

0 0 0 0 0 0 0 0 0

0 1 0 0 1 0 0 0 1

1 0 0 1 0 0 0 1 0

1 1 0 1 1 0 0 1 1

1 0 0 0 1 0 0

1 0 1 0 1 0 1

1 1 0 0 1 1 0

1 1 1 0 1 1 1

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

▲ Table 3.9

3.2.3 The function of the six logic gates

NOT gate

▲ Figure 3.23 NOT gate

Description

The output, X, is 1 if the input A is NOT 1

How to write this

X = NOT A (logic notation)

X = A

–

(Boolean algebra)

Truth table

Input Output

A X

0 1

1 0

▲ Table 3.10

457591_03_CI_AS & A_Level_CS_068-106.indd 90 02/05/19 12:36 PM

3.2 Logic gates and logic circuits

AND gate

▲ Figure 3.24 AND gate

Description

The output, X, is 1 if input A is 1 and input B

is 1

How to write this

X = A AND B (logic notation)

X = A.B (Boolean algebra)

Truth table

Inputs Output

▲ Table 3.11

OR gate

▲ Figure 3.25 OR gate

Description

The output, X, is 1 if input A is 1 or input B

is 1

How to write this

X = A OR B (logic notation)

X = A + B (Boolean algebra)

Truth table

Inputs Output

▲ Table 3.12

NAND gate (NOT AND)

▲ Figure 3.26 NAND gate

Description

The output, X, is 1 if input A is NOT 1 or

input B is NOT 1

How to write this

X = A NAND B (logic notation)

X = A.B (Boolean algebra)

Truth table

Inputs Output

▲ Table 3.13

NOR gate (NOT OR)

▲ Figure 3.27 NOR gate

Description

The output, X, is 1 if:

input A is NOT 1 and input B is NOT 1

How to write this

X = A NOR B (logic notation)

X = A + B (Boolean algebra)

Truth table

Inputs Output

▲ Table 3.14

457591_03_CI_AS & A_Level_CS_068-106.indd 91 02/05/19 12:36 PM

3 Hardware

XOR gate

▲ Figure 3.28 XOR gate

Description

The output, X, is 1 if (input A is 1 AND input B

is NOT 1) OR (input A is NOT 1 AND input B

is 1)

How to write this

X = A XOR B (logic notation)

X = (A.B) + (A.B) (Boolean algebra)

(Note: this is sometimes written as:

(A + B) . A.B)

Truth table

Inputs Output

A B X

0 0 0

0 1 1

1 0 1

1 1 0

▲ Table 3.15

EXTENSION ACTIVITY 3G

Using truth tables show that X = (A.B) + (A.B) and X = (A + B) . A.B both

represent the XOR logic gate.

You will notice, in the Boolean algebra, three new symbols.

» A dot (.) represents the AND operation (it can be written as

∧

» A plus sign (+) represents the OR operation (it can be written as ∨).

» A dash above a letter (for example, A) represents the NOT operation.

3.2.4 Logic circuits

When logic gates are combined to carry out a particular function, such as

controlling a robot, they form a logic circuit.

The output from the logic circuit is checked using a truth table. The following

three examples show how to:

» produce a truth table

» design a logic circuit from a given logic statement/Boolean algebra

» design a logic circuit to carry out an actual safety function.

Produce a truth table for the following logic circuit (note the use of at junctions):

part 1 part 2 part 3

Example 3.1

457591_03_CI_AS & A_Level_CS_068-106.indd 92 02/05/19 12:36 PM

3.2 Logic gates and logic circuits

Solution

There are three inputs to this logic circuit; therefore, there will be eight possible

binary values which can be input.

To show step-wise how the truth table is produced, the logic circuit has been split

up into three parts and intermediate values are shown as P, Q and R.

Part 1

This is the first part of the logic circuit; the first task is to find the intermediate

values P and Q.

The value of P is found from the AND gate where the inputs are A and B. The

value of Q is found from the NOR gate where the inputs are B and C. An

intermediate truth table is produced:

Inputs Outputs

A B C P Q

0 0 0 0 1

0 0 1 0 0

0 1 0 0 0

0 1 1 0 0

1 0 0 0 1

1 0 1 0 0

1 1 0 1 0

1 1 1 1 0

Part 2

The second part of the logic circuit has P and Q as inputs and the intermediate

output, R.

This produces the following intermediate

truth table (Note: even though there are

only two inputs to the logic gate, we have

generated eight binary values in Part 1

and these must all be used in this second

truth table).

Inputs Output

P Q R

0 1 1

0 0 0

0 1 1

0 0 0

1 0 1

457591_03_CI_AS & A_Level_CS_068-106.indd 93 26/04/19 7:27 AM

3 Hardware

Part 3

The final part of the logic circuit has R

and C as inputs and the final output, X.

This gives the third intermediate truth

table.

Putting all three intermediate truth

tables together produces the final truth

table which represents the original

logic circuit.

Inputs Intermediate values Output

A B C P Q R X

0 0 0 0 1 1 1

0 0 1 0 0 0 1

0 1 0 0 0 0 0

0 1 1 0 0 0 1

1 0 0 0 1 1 1

1 0 1 0 0 0 1

1 1 0 1 0 1 1

1 1 1 1 0 1 0

ACTIVITY 3B

Produce truth tables for each of the following logic circuits. You are advised to split them up into

intermediate parts to help eliminate errors.

a) b)

d) e)

Inputs Output

R C X

1 0 1

0 1 1

0 0 0

0 1 1

1 0 1

0 1 1

1 0 1

1 1 0

457591_03_CI_AS & A_Level_CS_068-106.indd 94 26/04/19 7:27 AM

3.2 Logic gates and logic circuits

A safety system uses three inputs to a logic circuit. An alarm, X, sounds if input A

represents ON and input B represents OFF, or if input B represents ON and input C

represents OFF.

Produce a logic circuit and truth table to show the conditions which cause the

output X to be 1.

Example 3.2

Solution

The first thing to do is to write down the logic statement representing the

scenario in this example. To do this, it is necessary to recall that ON = 1 and OFF

= 0 and also that 0 is considered to be NOT 1.

So, we get the following logic statement:

X = 1 if (A = 1 AND

B = NOT 1)

OR (B = 1 AND

C = NOT 1)

this equates to

A is ON and B

is OFF

the two parts are

connected by the

OR gate

this equates to

B is ON AND

C is OFF

Part 1 Part 2 Part 3

This statement can also be written in Boolean algebra as:

(A.B) + (B.C)

The logic circuit is made up of three parts as shown in the logic statement. We

will produce the logic gate for the Part 1 and Part 3, then join both parts together

with the OR gate.

Part 1 Part 3

Now, combining both parts with Part 2 (the OR gate) gives us:

Part 1

Part 2

Part 3

There are two ways to produce the truth table.

l Trace through the logic circuit using the method described in Example 3.1.

l Use the original logic statement; this allows you to check that your logic

circuit is correct.

457591_03_CI_AS & A_Level_CS_068-106.indd 95 26/04/19 7:27 AM

3 Hardware

ACTIVITY 3C

Draw the logic circuits and complete the truth tables for these logic

statements and Boolean algebra statements.

a) X = 1 if (A = 1 OR B = 1) OR (A = 0 AND B = 1)

b) Y = 1 if (A = 0 AND B = 0) AND (B = 0 OR C = 1)

c) T = 1 if (switch K is ON or switch L is ON) OR (switch K is ON and switch M

is OFF) OR (switch M is ON)

d) X = (A.B) + (B.C)

e) R = 1 if (switch A is ON and switch B is ON) AND (switch B is ON or switch

C is OFF)

We will use the second method in this example.

Inputs Intermediate values Output

A B C (A=1 AND

B=NOT 1)

(B=1 AND

C=NOT 1)

0 0 0 0 0 0

0 0 1 0 0 0

0 1 0 0 1 1

0 1 1 0 0 0

1 0 0 1 0 1

1 0 1 1 0 1

1 1 0 0 1 1

1 1 1 0 0 0

A wind turbine has a safety system which uses three inputs to a logic circuit. A

certain combination of conditions results in an output, X, from the logic circuit

being equal to 1. When the value of X = 1, the wind turbine is shut down.

The following table shows which parameters are being monitored and form the

three inputs to the logic circuit.

Parameter description Parameter Binary value Description of condition

turbine speed S 0

turbine speed ≤ 1000 rpm

turbine speed > 1000 rpm

bearing temperature T 0

bearing temperature ≤ 80 °C

bearing temperature > 80 °C

wind velocity W 0

wind velocity ≤ 120 kph

wind velocity > 120 kph

The output, X, will have a value of 1 if any of the following combination of

conditions occur:

l either turbine speed ≤ 1000 rpm and bearing temperature > 80 °C

l or turbine speed > 1000 rpm and wind velocity > 120 kph

l or bearing temperature ≤ 80 °C and wind velocity > 120 kph

Example 3.3

457591_03_CI_AS & A_Level_CS_068-106.indd 96 26/04/19 7:27 AM

3.2 Logic gates and logic circuits

Design the logic circuit and complete the truth table to produce a value of X = 1

when either of the three conditions occur.

Solution

This is a different type of problem to those covered in Examples 3.1 and 3.2. This

time, a real situation is given and it is necessary to convert the information into a

logic statement and then produce the logic circuit and truth table. It is advisable in

problems as complex as this to produce the logic circuit and truth table separately

(based on the conditions given) and then check them against each other to see if

there are any errors.

Stage 1

The first thing to do is to convert each of the three statements into logic

statements. Use the information given in the table and the three condition

statements to find how the three parameters S, T and W, are linked. We usually

look for the key words AND, OR and NOT when converting actual statements

into logic.

We end up with these three logic statements:

① turbine speed  1000 rpm and bearing temperature > 80 °C

logic statement: (S = NOT 1 AND T = 1)

② turbine speed > 1000 rpm and wind velocity > 120 kph

logic statement: (S = 1 AND W = 1)

③ bearing temperature  80 °C and wind velocity > 120 kph

logic statement: (T = NOT 1 AND W = 1)

Stage 2

This produces three intermediate logic circuits:

①

②

③

Each of the three original statements were joined together by the word OR. So,

we need to join all of the three intermediate logic circuits by two OR gates to get

the final logic circuit.

We will start by joining ① and ② together using an OR gate.

457591_03_CI_AS & A_Level_CS_068-106.indd 97 26/04/19 7:27 AM

3 Hardware

Now, we connect this to logic circuit ③ to obtain the final logic circuit.

The final part is to produce the truth table. We will do this using the original

logic statement, since this method allows an extra check to be made on the final

logic circuit.

There were three parts to the problem, so the truth table will first evaluate each

part. Then, by applying OR gates, as shown below, the final value, X, is obtained:

① (S = NOT 1 AND T = 1)

② (S = 1 AND W = 1)

③ (T = NOT 1 AND W = 1)

We find the outputs from ① and ② and then OR these two outputs to obtain a

new intermediate, which we will label part ④.

We then OR parts ③ and ④ together to get the value of X.

Inputs Intermediate values Output

A B C

①

(S=NOT 1

AND T=1)

②

(S=1 AND

W=1)

③

(T=NOT 1

AND W=1)

④

0 0 0 0 0 0 0 0

0 0 1 0 0 1 0 1

0 1 0 1 0 0 1 1

0 1 1 1 0 0 1 1

1 0 0 0 0 0 0 0

1 0 1 0 1 1 1 1

1 1 0 0 0 0 0 0

1 1 1 0 1 0 1 1

ACTIVITY 3D

There are two scenarios described below. In each case, produce the logic

circuit and complete a truth table to represent the scenario.

a) A chemical process is protected by a logic circuit. There are three

inputs to the logic circuit representing key parameters in the chemical

process.

An alarm, X, will give an output value of 1 depending on certain

conditions in the chemical process.

457591_03_CI_AS & A_Level_CS_068-106.indd 98 26/04/19 7:27 AM

3.2 Logic gates and logic circuits

3.2.5 Logic circuits in the real world

The design of logic circuits is considerably more complex than has, so far, been

described. We have discussed some of the fundamental theories, providing

sufficient coverage of the Cambridge International A Level syllabus. However, it

is worth discussing some of the more advanced aspects of logic circuit design,

to strengthen understanding.

Electronics companies need to consider the cost of components, ease of

fabrication and time constraints when designing and building logic circuits.

Ways electronics companies review logic circuit design include:

» using ‘off-the-shelf’ logic units and building up the logic circuit as a number

of ‘building blocks’

» simplifying the logic circuit as far as possible; this may be necessary

where room is at a premium (for example, building circuit boards for use in

satellites for space exploration).

This table describes the process conditions being monitored.

Parameter

description Parameter

Binary

value Description of condition

chemical

reaction rate

R 0

reaction rate < 40 mol/l/sec

reaction rate  40 mol/l/sec

process

temperature

T 0

temperature > 115 °C

temperature  115 °C

concentration

of chemicals

C 0 concentration = 4 mol

concentration > 4 mol

An alarm, X, will generate the value 1 if:

either reaction rate < 40 mol/l/sec

or concentration > 4 mol AND temperature > 115 °C

or reaction rate  40 mol/l/sec AND temperature > 115 °C.

b) A power station has a safety system controlled by a logic circuit. Three

inputs to the logic circuit determine whether the output, S, is 1.

When S = 1 the power station shuts down.

The following table describes the conditions being monitored.

Parameter

description Parameter

Binary

value Description of condition

gas temperature G 0

gas temperature  160 °C

gas temperature > 160 °C

reactor pressure R 0

reactor pressure  10 bar

reactor pressure > 10 bar

water

temperature

W 0

water temperature  120 °C

water temperature > 120 °C

Output, S, will generate a value of 1, if:

either gas temperature > 160 °C AND water temperature  120 °C

or gas temperature  160 °C AND reactor pressure > 10 bar

or water temperature > 120 °C AND reactor pressure > 10 bar.

457591_03_CI_AS & A_Level_CS_068-106.indd 99 26/04/19 7:27 AM

100

3 Hardware

Using logic ‘building blocks’

One common ‘building block’ is the NAND gate. It is possible to build up any

logic gate, and therefore any logic circuit, by simply linking together a number

of NAND gates, such as:

» the AND gate

▲ Figure 3.29 AND gate made from NAND gates

» the OR gate

▲ Figure 3.30 OR gate made from NAND gates

» the NOT gate

▲ Figure 3.31 NOT gate made from NAND gates

ACTIVITY 3E

1 By drawing the truth tables, show that the three logic circuits shown

above can be used to represent AND, OR and NOT gates.

2 a) Show how the following logic circuit could be built using NAND gates only.

Complete truth tables for both logic circuits to show that they produce

identical outputs.

b) Show how the XOR gate could be built from NAND gates only.

Complete a truth table for your final design to show that it produces

the same output as a single XOR gate.

3 By drawing a truth table, discover which single logic gate has the same

function as the following logic circuit made up of NAND gates only.

457591_03_CI_AS & A_Level_CS_068-106.indd 100 26/04/19 7:27 AM

101

3.2 Logic gates and logic circuits

Simplification of logic circuits

The second method involves the simplification of logic circuits. By reducing

the number of components, the cost of production can be less. This can also

improve reliability and make it easier to trace faults if they occur. This is

covered in more depth in Chapter 15.

3.2.6 Multi-input logic gates

This section looks at logic gates with more than two inputs (apart from the NOT

gate). Students are not expected to answer questions about multi-input logic

gates at Cambridge International AS Level, but this information is included here

for completeness and for those with an electronics background. This is intended to

complete the picture for interested students who may have seen multi-input gates

in other textbooks, or online, and it leads neatly into topics covered in Chapter 15.

Logic gates (apart from the NOT gate) can have more than two inputs. While it is

still acceptable to use two-input logic gates, it is worth considering the multi-

input option when designing logic circuits; they can simplify the overall result.

Multi-input AND gates

is the same as

▲ Figure 3.32 Multi-input AND gate

Both sets of AND gates have the output A.B.C and they share identical truth tables.

Inputs Output

A B C A.B.C

0 0 0 0

0 0 1 0

0 1 0 0

0 1 1 0

1 0 0 0

1 0 1 0

1 1 0 0

1 1 1 1

▲ Table 3.16

EXTENSION ACTIVITY 3H

By drawing a truth table, show which single logic gate has the same function

as the logic circuit drawn below.

457591_03_CI_AS & A_Level_CS_068-106.indd 101 4/30/19 7:48 AM

102

3 Hardware

Now consider the following:

is the same as

▲ Figure 3.33 4-input AND gate

Both sets of AND gates have the output A.B.C.D and they share identical truth tables.

Inputs Output

A B C D A.B.C.D

0 0 0 0 0

0 0 0 1 0

0 0 1 0 0

0 0 1 1 0

0 1 0 0 0

0 1 0 1 0

0 1 1 0 0

0 1 1 1 0

1 0 0 0 0

1 0 0 1 0

1 0 1 0 0

1 0 1 1 0

1 1 0 0 0

1 1 0 1 0

1 1 1 0 0

1 1 1 1 1

▲ Table 3.17

Multi-input OR gates

is the same as

▲ Figure 3.34 Multi-input OR gate

Both sets of OR gates have the output A + B + C and they share identical truth tables.

Inputs Output

A B C A + B + C

0 0 0 0

0 0 1 1

0 1 0 1

0 1 1 1

1 0 0 1

1 0 1 1

1 1 0 1

1 1 1 1

▲ Table 3.18

457591_03_CI_AS & A_Level_CS_068-106.indd 102 26/04/19 7:27 AM

103

3.2 Logic gates and logic circuits

Now consider the following:

is the same as

▲ Figure 3.35 4-input OR gate

Both sets of OR gates have the output A + B + C + D and they share identical

truth tables.

Inputs Output

A B C D A + B + C + D

0 0 0 0 0

0 0 0 1 1

0 0 1 0 1

0 0 1 1 1

0 1 0 0 1

0 1 0 1 1

0 1 1 0 1

0 1 1 1 1

1 0 0 0 1

1 0 0 1 1

1 0 1 0 1

1 0 1 1 1

1 1 0 0 1

1 1 0 1 1

1 1 1 0 1

1 1 1 1 1

▲ Table 3.19

ACTIVITY 3F

1 a) Draw the following multi-input NAND gate using two-input NAND

gates only:

b) Construct the truth tables for the above 4-input NAND gate and for

your circuit drawn in part a). Confirm that they are identical.

2 a) Draw the following multi-input NOR gates using two-input NOR gates only.

b) Construct the truth tables for the above 3-input NOR gate and for your

equivalent circuit drawn in part a).

Confirm they are identical.

457591_03_CI_AS & A_Level_CS_068-106.indd 103 26/04/19 7:27 AM

104

3 Hardware

c) Construct the truth tables for the above 4-input NOR gate and for your

equivalent circuit drawn in part a).

Confirm they are identical.

3 Confirm that the following two logic circuits are identical by constructing

the truth tables for each circuit.

1 a) Many mobile phone and tablet manufacturers are moving to OLED screen

technology.

Give three reasons why this is happening. [3]

b) A television manufacturer makes the following advertising claim:

‘Our OLED screens allow the user to enjoy over one million vivid colours in

true-to-life vision.’

Comment on the validity of this claim. [4]

2 a) A company is developing a new games console. The game will be stored on a

ROM chip once the program to run the new game has been fully tested and

developed.

i) Give two advantages of putting the game’s program on a ROM chip. [2]

ii) Explain why the manufacturers would use an EPROM chip during

development. [2]

iii) The manufacturers are also using RAM chips on the internal circuit board.

Explain why they are doing this. [2]

iv) The games console will have four USB ports.

Apart from the need to attach games controllers, give reasons why USB

ports are incorporated. [2]

b) During development of the games console the plastic parts are being made by

a 3D printer.

Give two reasons why the manufacturer would use 3D printers. [2]

3 An air conditioning unit in a car is being controlled by a microprocessor and a

number of sensors.

a) Describe the main differences between control and monitoring of

a process. [2]

b) Describe how the sensors and microprocessor would be used to control the

air conditioning unit in the car.

Name at least two different sensors that might be used and explain the role of

positive feedback in your description.

You might find drawing a diagram of your intended process to

be helpful. [6]

End of chapter

questions

457591_03_CI_AS & A_Level_CS_068-106.indd 104 26/04/19 7:27 AM

105

3.2 Logic gates and logic circuits

4 The nine stages in printing a page using an inkjet printer are shown below.

They are not in the correct order.

Write the letters A to I so that the stages are in the correct order. [9]

A The data is then sent to the printer and it is stored in a temporary memory

known as a printer buffer.

B As the sheet of paper is fed through the printer, the print head moves

from side to side across the paper printing the text or image. The four ink

colours are sprayed in their exact amounts to produce the desired final

colour.

C The data from the document is sent to a printer driver.

D Once the printer buffer is empty, the printer sends an interrupt to the

processor in the computer, which is a request for more data to be sent to the

printer. The whole process continues until the whole of the document has

been printed.

E The printer driver ensures that the data is in a format that the chosen printer

can understand.

F At the end of each full pass of the print head, the paper is advanced very

slightly to allow the next line to be printed. This continues until the whole

page has been printed.

G A check is made by the printer driver to ensure that the chosen printer is

available to print (is it busy? is it off line? is it out of ink? and so on).

H If there is more data in the printer buffer, then the whole process from stage

5 is repeated until the buffer is finally empty.

I A sheet of paper is then fed into the main body of the printer, where a sensor

detects whether paper is available in the paper feed tray – if it is out of

paper (or the paper is jammed) then an error message is sent back to

the computer.

5 a) There are two types of RAM: dynamic RAM (DRAM) and static RAM

(SRAM). Five statements about DRAM and RAM are shown below. Copy

the diagram below and connect each statement to the appropriate type of

RAM. [5]

Statement Type of RAM

requires the data to be refreshed periodically in

order to retain data

has more complex circuitry

DRAM

does not need to be refreshed as the circuit holds

the data as long as the power supply is on

requires higher power consumption which is

significant when used in battery-powered devices

SRAM

used predominantly in cache memory of

processors where speed is important

b) Give three differences between RAM and ROM. [3]

➔

457591_03_CI_AS & A_Level_CS_068-106.indd 105 26/04/19 7:27 AM

106

3 Hardware

c) DVD-RAM and flash memory are two examples of storage devices.

Describe two differences in how they operate. [2]

Cambridge International AS & A Level Computer Science 9608

Paper 13 Q4 June 2015

6 a) Three digital sensors, A, B and C, are used to monitor a process. The outputs

from the sensors are used as the inputs to a logic circuit. A signal, X, is output

from the logic circuit:

logic

circuit

output X

Output, X, has a value of 1 if either of the following two conditions occur:

– Sensor A outputs the value 1 OR sensor B outputs the value 0.

– Sensor B outputs the value 1 AND sensor C outputs the value 0.

Draw a logic circuit to represent these conditions. [5]

b) Copy and complete the truth table for the logic circuit described in

part a). [4]

A B C working space X

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

c) Write a logic statement that describes the following logic circuit. [3]

Cambridge International AS & A Level Computer Science 9608

Paper 13 Q6 June 2015

457591_03_CI_AS & A_Level_CS_068-106.indd 106 26/04/19 7:27 AM